How often should you replace your Aquarium Metal Halide Light Bulbs?
Because metal halides light bulbs produce more lumens per watt than other types of aquarium illumination, many reef hobbyists prefer this type of lighting. We suggest that the bulbs be replaced every 10-12 months. Even though the lamp will produce light for much longer (10,000 hr +), however, there is a significant lumens loss with the lamp over time (up to 50% in just 5,000 hours of use). This can put a strain on your flora, coral and other live stock that depend on the light for survival.
Replacement metal halide reef lamps can be used longer (12-18 months) when used with an electronic ballast.